This invention relates generally to a method for reducing nitrogen oxides (NO.sub.x) produced during combustion in the engines of supersonic aircraft and more particularly concerns a method involving the injection of a reducing agent into the exhaust or the combustor of supersonic aircraft engines.
A new generation of supersonic transport, referred to as high speed civil transport (HSCT), is currently under development. As now envisioned, HSCT will be capable of cruising at approximately 2.5 Mach, have transpacific range and carry approximately 300 passengers. Obstacles to the introduction of such large supersonic aircraft include the production of stratospheric nitrogen oxides (NO.sub.x), sonic boom and excessive noise. Nitrogen oxides have a lasting deleterious effect on the environment, unlike the sonic boom and noise which are temporary. The presence of NO.sub.x creates a number of environmental problems, not the least of which being the destruction of ozone in the stratosphere, thereby depleting the natural shield against solar ultraviolet radiation. Consequently, there is a great need to reduce the amount of NO.sub.x produced by HSCT.
Due to their high cruising speeds, the HSCT engines will operate at combustor inlet temperatures significantly higher than subsonic aircraft. The high temperature levels are in part due to ram compression of high speed air entering the engines. The ensuing high combustion temperature leads to copious production of NO.sub.x by the thermal mechanism. According to well-supported combustion theory, NO.sub.x produced by the thermal mechanism is due to reactions between atmospheric nitrogen and the free radicals which accompany the combustion process. Thermal NO.sub.x production becomes significant at temperatures above about 3000.degree. F. Since takeoff and cruise performance must not be impaired, attempts (such as lean premixed, prevaporized combustion) to reduce the combustion temperature to a level below which thermal NO.sub.x production occurs are not seen as an assured solution to the NO.sub.x problem. Thus, it is believed that the best approach to minimizing NO.sub.x production recognizes that NO.sub.x will be produced in the combustion process and must later be reduced.